The contribution of immigrants to the changing rate of pulmonary tuberculosis (TB) has been observed in many regions of the world, such as Europe [1], USA [2], Canada [3], Australia [4], and Scandinavian countries [5]. Currently, the proportion of immigrants among prevalent TB cases exceeds 50% in several European countries [6], and in United States [7]. In Kuwait, TB incidence showed a decline from 1965 to 1989. However, from 1989 to 1999, there was a steady increase in the TB incidence among both nationals and non-nationals suggestive of Mycobacterium tuberculosis transmission from non-nationals to nationals since a large proportion of migrants from South Asian and South-East Asian countries live and work in Kuwaiti homes as domestic workers [8]. This reversal of the observed TB trend in Kuwait may be explained in part by the disruption of TB control program following first Gulf War in 1990 [8], and by the concomitant increase in the proportion of migrants from high TB burden countries [9]. The epidemiologic importance of migration from high TB incidence to low TB incidence countries has been recognized for several years; the main countermeasure has been the implementation of screening programs for immigrants at the time of arrival, and providing required chemotherapy [10, 11].

Trend analysis of TB incidence may help to identify its risk factors and target interventions to prevent it. But it is also important to identify possible seasonal pattern in the disease incidence, the knowledge of which may be used; i. to predict the future magnitude of the health problem; ii. to develop an effective public health program, and iii. to set objectives and utilize available resources more effectively [12]. TB is not widely known to have any seasonal patterns, but few studies have shown variable periods of peak seasonality in TB incidence/case notification rates in late winter to early spring in South Africa [13], during summer in UK [14] and Hong Kong [15], during summer and autumn in Spain [12] and Japan [16]. Relatively recently, it was demonstrated that TB diagnosis peaked between April and June and reached a nadir between October and December in northern India and magnitude of seasonal variation had significant positive correlation with TB case rates [17]. However, to our knowledge, no studies so far have described TB seasonality in Middle East. In Kuwait; there is evidence of seasonal variation in respiratory disease related morbidity and mortality which peaks during winter [18]. Nonetheless, no published data on any seasonal pattern in positive TB diagnosis in Kuwaiti population specifically among migrants from high TB burden countries are available. Therefore, this study examined possible seasonal variation in pulmonary TB cases detected among migrant workers entered Kuwait between January 1, 1997 and December 31, 2006.

The findings of this study suggested that there was a spring peak (late April) in TB cases detected among migrants entering Kuwait from high TB burden countries. The estimated amplitude of seasonal variation in the number of TB cases was 20.4% from the annual mean. The observed amplitude of seasonal variation was substantial enough to be explained by monthly variations in the number of TB cases detected among these migrants. In the context of TB diagnosis/case notification rates, few studies have reported variable peaks of seasonality from different parts of the world. For example, in the pre-antibiotic era, TB related mortality was higher in the late winter and early spring than at any other time of the year [25]. Relatively few recent studies have shown a similar seasonal pattern in TB notification rates [13, 26, 27]. Also, peak seasonality has been reported during summer in UK [14], and Hong Kong [15], during summer and autumn in Spain [12], Japan [16], and April to June in India [17]. Exactly, why TB diagnosis/case notification rates might vary by season is unknown and the specific contributions of a variety of climatic and meteorological seasonal changes are uncertain. However, it has been suggested that winter indoor crowding in poorly ventilated settings could lead to increased M. tuberculosis transmission, which then manifests itself 3–6 months later [13]. As noted above, the preclinical period, from exposure to clinical onset, may be of several weeks, which may in part, explain variable seasonal peaks in different geographic regions [12, 13, 16].

In this study, all of the detected TB cases were adults, and most of the TB cases in adults are considered to be the results of reactivation of latent M. tuberculosis infection. In the absence of HIV infection or immunosuppressive therapy, such TB cases resulting from re-activation of latent M. tuberculosis infection are attributed to poor nutrition and low socio-economic status. Although the exact mechanism of this re-activation of M. tuberculosis infection remains unclear [28], yet cell mediated immunity in M. tuberculosis infection seems to play role because of circannual variation in lymphocyte subsets. The seasonal changes in the absolute numbers and ratios of T helper and T suppressor cells could possibly alter cell mediated immunity that controls host response to M. tuberculosis infection. Nevertheless, the factors that regulate the seasonal changes in T cell subset numbers or function remain unknown [29]. It has been argued however, that in winter and spring, the viral infections like flu, are more frequent and cause immunological deficiency leading to re-activation of M. tuberculosis infection [12]. Furthermore, a probable link between impaired host immunological defence due to vitamin D deficiency and the re-activation of latent M. tuberculosis infection has been hypothesized [30–32]. The principal source of vitamin D is ultraviolet radiation from sunlight, and that plasma concentrations of vitamin D have a striking seasonal variation with peak levels after the summer and lowest levels in the spring [31, 33]. Also, a significant trend of increasing TB risk with increased vitamin D deficiency due to low frequency of meat or fish consumption among vegetarian Asians has been reported [28]. Since most of the migrants entered in Kuwait during the study period came from South Asia. Therefore, period for the peak seasonality in late April in this study corroborated the findings of reported seasonality in April – June in northern India [17]. Furthermore, the researchers of the aforementioned study have suggested that winter transmission of M. tuberculosis due to increased indoor activity and/or vitamin D deficiency leading to re-activation of latent infection may have been the bases for the observed TB seasonality in northern India [17, 34]. Perhaps similar biological phenomena may have been associated with significant seasonality in late April among migrants in our study. Also, it may well be that the migrants from India have a very high proportion of TB and arrives mainly in winter months while migrants from Sri Lanka and other low burden countries have a low proportion of TB and arrive mainly in summer. Such workers' recruitment pattern may have potentially contributed in seasonal variation in pulmonary TB in migrants entering Kuwait. However, we did not have season-specific recruitment data nested within the countries of origin of these migrants to support this contention. Nonetheless, exact biological basis for period of peak seasonality during April in this population needs further investigations.

Taken together, the results of this and previous studies seem to corroborate the evidence for seasonality in detection of TB cases, which peaks in late April in this migrant population. This regularity of peak seasonality in TB cases may prove useful to institute measures that warrant a better attendance of migrants. Public health authorities may consider re-allocation of resources in the period of peak seasonality to minimize M. tuberculosis infection risk to close contacts in this and comparable settings in the region having similar influx of migrants from high TB burden countries. Epidemiological surveillance for the TB risk in the migrants in subsequent years and required chemotherapy of detected cases may contribute in global efforts to control this public health menace.